Upon removing a silicon compound film from a semiconductor silicon substrate in an etching system, the silicon compound film is generally subjected to etching with a phosphoric acid solution of high temperature and high concentration. The etching solution (phosphoric acid solution) employed in the above-described etching has been recirculated and used many times. After each etching, the silicon component remains as a nonvolatile silicon compound in the phosphoric acid solution, and under this condition, the next etching is conducted. The concentration of silicon contained as the silicon compound in the phosphoric acid solution, therefore, varies in every etching. As a result of such variations in the concentration of silicon in the etching solution, changes take place in the etch characteristics of silicon compound films to be etched as will be described below.
Firstly, an increase in the concentration of silicon in a phosphoric acid solution leads to a reduction in the etch rate of a silicon compound film. With a view to inhibiting such a reduction in etch rate, it has been proposed to have a solute (silicon component) in a phosphoric acid solution, which is under use for etching, collectively precipitated so that the life of the phosphoric acid solution can be prolonged (for example, JP-A-2002-299313 and JP-A-11-293479).
According to an investigation by the present inventors, the above-described reduction in etch rate has been found to substantially differ in percent reduction depending on the silicon compound of the silicon compound film to be etched. Comparing a silicon nitride film with a silicon oxide film, for example, there is a difference as will be described below. When the concentration of silicon in a phosphoric acid solution increases from 0 ppm to 50 ppm under given etching conditions, the etch rate of the silicon oxide film is reduced significantly by 75 to 85% as opposed to the percent reduction of the etch rate of the silicon nitride film which is as little as 10 to 20% or so. It is, therefore, very important for uniform etching to accurately grasp the concentration of silicon in a phosphoric acid solution, which is under use as an etching solution in an etching system, and to control the concentration of silicon.
A control method, which has conventionally been applied to a phosphoric acid solution under use in an etching system in operation, is to periodically replace the recirculated and used phosphoric acid solution lot by lot. Namely, the phosphoric acid solution is periodically replaced to prevent the concentration of silicon in the phosphoric acid solution, which is to be used for etching, from exceeding a tolerance. However, the solution life which is relied upon in the above control method has been derived from a rule of thumb, so that control in real sense, specifically the control of silicon concentration in a phosphoric acid solution under use for etching is not performed.
The above-described etching process is conducted with a phosphoric acid solution which does not contain much silicon. In contrast to this etching process, there is also an etching process in which with a view to making an improvement in the etch selectivity in the etching of a multilayer film, a wafer with a silicon compound film formed thereon is used to deliberately dissolve silicon in an etching solution and etching is then conducted in the phosphoric acid solution with silicon included therein beforehand. Similar to the above-described control method, the control of the life of the etching solution also relies upon a rule of thumb in this etching process. In this etching process, the concentration at which silicon is dissolved in the phosphoric acid solution beforehand is achieved by dissolving, in the above-described manner, silicon in the phosphoric acid solution under conditions derived from experiences. In other words, this etching process is not performed either while controlling whether or not a desired amount of silicon is actually dissolved in the phosphoric acid solution under use.
If the concentration of silicon is successively measured in a phosphoric acid solution under recirculation and use in an etching step of a silicon compound film with the phosphoric acid solution, control of the silicon concentration in a phosphoric acid solution, in real sense, becomes feasible. Further, the control of the concentration of silicon actually contained in the phosphoric acid solution under use as an etching solution directly leads to the control of etch rate and etch selectively as described above.
Under the current circumstances, however, the control of the life of a phosphoric acid solution (the control of the concentration of silicon in a phosphoric acid solution) is solely performed under a rule of thumb as descried above. This is attributed to the existence of no known method or equipment for the measurement of the concentration of silicon contained in a phosphoric acid solution under recirculation and use, which can be used online for (in other words, can be used for automated control of) an etching system that subjects a silicon compound film, which is formed on a semiconductor silicon substrate, to etching with the phosphoric acid solution at high temperature. A description will hereinafter be made about the current circumstances.
Methods, which are commonly employed upon performing an analysis of the concentration of silicon in a phosphoric acid solution, include ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) based on JIS-K0116 and spectral analysis methods such as an atomic absorption analysis based on JIS-K0121. The use of these analytical methods permits precise analyses at very high accuracy.
Adoption of such a method, however, requires an emitter for making an analysis target emit light, a spectrometer for dispersing the light, a detector for detecting the thus-dispersed light, and the like, and therefore, requires a complex and large etching system. Moreover, the phosphoric acid solution under use as an etching solution in the etching system, said phosphoric acid solution being a measurement target, is high in concentration and also high in temperature. For the application of such an analytical method as described above, it is necessary to conduct pretreatment of a measurement sample before the measurement such as cooling it down to around room temperature and subsequently diluting the same. It is, therefore, very difficult to incorporate such analysis equipment in an etching system and to perform measurement continuously (with time). Moreover, such analysis equipment is costly not only in its own price but also in its maintenance. There is, accordingly, a problem in this respect too.